a) Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to an active matrix type LCD wherein each liquid crystal cell has a switching thin film transistor.
b) Description of the Related Art
An LCD has a liquid crystal layer sandwiched between a pair of plates having electrodes. The state of liquid crystal molecules is controlled by voltage applied between two electrodes facing each other, to regulate light transmission.
For a large display screen, it is convenient that a number of pixels are disposed in the screen area in a matrix form and are driven by switching elements. To this end, an active matrix type LCD has been used in which row and column lines are wired in the screen area, and a thin film transistor is formed at each cross point between row and column lines to drive the pixel electrode.
For example, video information is supplied to a column driver having as many output lines as the number of columns, and a row driver is driven to sequentially scan row lines to supply video information one line after another.
A drive time period per one line is T/n, where n is the number of rows within one frame and T is one frame display time. The more the number of rows of a frame, the shorter the drive time period per each pixel. If a voltage built up within a pixel changes during an off-period of a switching or driver transistor, the quality of a displayed image will be degraded. In order to maintain a good image quality, some approaches have been proposed, for example, connecting a capacitor to each pixel.